The present invention relates to hydraulic systems including axial piston pumps and, in particular, relates to closed center hydrostatic drive systems with variable displacement axial piston pumps.
Closed center hydrostatic drives with variable displacement axial piston pumps require a source of pressure to provide a control pressure for the servo pistons which control the pump swashplate angle. In many such systems, the charging pump pressure is utilized in conjunction with large servo pistons for controlling swashplate angle. Other such systems utilize the output or system pressure as this control pressure with reduced size servos. This system pressure is normally adequate, except when the swashplate is in a neutral or low pump output position or when the pump is supplying fluid to an overrunning load. In these latter situations, the system pressure may be less than what is required for adequate control of the servo pistons or, in otherwords, less than what is required to overcome the friction which tends to resist changes in the position of the swashplate and piston inertia which tends to increase the swashplate angle.